1. Technical Field
The present invention generally relates to electrically driven solid state lasers and, more particularly, to an apparatus and method for producing high average power laser light from a distributed series of fiber amplifiers.
2. Discussion
Large scale electrically driven solid state lasers are currently used in numerous military and commercial applications. At power levels greater than 10 Watts (W), fiber lasers currently provide the most efficient generation of optical power from electrical sources with diffraction limited brightness. Unfortunately, individual fiber elements are currently limited to the 100 Watt level due to the small mode volume within the fiber.
To overcome the limitations of individual fiber elements, arrays of fibers are employed to generate multi-kilowatt or megawatt optical powers. There are currently two approaches of integrating fiber arrays into a unified coherent output. The first approach involves phasing individual fibers. The second approach involves wavelength division multiplexing.
While each of these approaches has merits, each also has disadvantages. As such, it would be desirable to provide an apparatus and method which combines these approaches so that either phasing, wavelength division multiplexing or both can be incorporated into an optical fiber array depending on the specific application. In addition, it would be desirable to provide a combined approach which allows the synthesis of coherent temporal wave forms of arbitrary shape.
The above and other objects are provided by an apparatus including a source of spectrally dispersed seed wavelengths optically coupled to an array of fibers. Diode pumps lasers are optically coupled to the array of fibers for amplifying the wavelengths through the array of fibers. A computer controlled feedback loop intercouples the array of fibers, the amplifier, the source of seed wavelengths, and/or phase modulators for maintaining the amplitude, phase and/or wavelengths in the array of fibers to desired levels. A compressor is optically coupled to an end of the array of fibers so as to receive and overlap the wavelengths from the individual fibers of the array.
In one embodiment of the present invention, the source of seed wavelengths comprises an array of wavelength controllable seed lasers. In another embodiment, the source of seed wavelengths comprises a short pulse laser and a stretcher which spatially separates the frequency components of the output of the short pulse laser. In yet another embodiment, the source of seed wavelengths includes a short pulse laser and a stretcher as well as a high speed phase modulator array and a low speed phase modulator array interposed between the stretcher and the array of fibers.
In still another embodiment of the present invention, the controlled feedback loop comprises a plurality of fiber taps optically coupled to the array of fibers for tapping wavelengths in the individual fibers of the array and a photodiode array optically coupled to the plurality of fiber taps for monitoring power levels in the individual fibers of the array. In another embodiment, the control feedback loop comprises a plurality of fiber taps optically coupled to the array of fibers for tapping wavelengths in the individual fibers of the array and an imaging spectrometer optically coupled to the plurality of fiber taps for monitoring the power levels and, if desired, wavelengths in the individual fibers of the array. In yet another embodiment, the controlled feedback loop includes fiber taps and an imaging spectrometer as well as a shearing interferometer array and a non-linear crystal array interposed between the plurality of fiber taps and the spectrometer for monitoring the power level, wavelengths and phase in the fibers of the array.